1. Field of the Invention
The present invention relates to a switching power supply unit having an overload protecting function and an overcurrent protecting function and to a semiconductor device used for the switching power supply unit.
2. Background Art
FIG. 8 is a circuit diagram of an example of conventional switching power supply units. In FIG. 8, reference numeral 130 denotes a semiconductor device for controlling the switching power supply. The semiconductor device 130 includes a switching device 101 and its control circuit.
The semiconductor device 130 is provided with the input terminal (DRAIN) of the switching device 101, an auxiliary power supply voltage input terminal (VCC), an internal circuit power supply terminal (VDD), a feedback signal input terminal (FB), the output terminal of the switching device 101, and the GND terminal (GND) of the control circuit as external input terminals.
Reference numeral 102 is a regulator which supplies the internal circuit power supply of the semiconductor device 130. The regulator is provided with a switch 102A, which flows a starting current into VCC, and a switch 102B which supplies the current from VCC to VDD.
Reference numeral 103 denotes a start-up constant current source which supplies a start-up circuit current. The constant current source supplies the starting current to VCC via the switch 102A during start up.
Reference numeral 107 denotes a start-up and shut-down circuit which controls the start up and the shut down of the semiconductor device 130. The start-up and shut-down circuit detects voltage at VCC and outputs a signal which stops the switching operation of the switching device 101 to a NAND circuit 105 when voltage at VDD is below a constant voltage.
Reference numeral 106 denotes a drain current detection circuit which detects a current flowing into the switching device 101 through the detection of the on voltage at the switching device 101 generated by the product of the current flowing into the switching device 101 and the on resistance of the switching device 101 and which converts the detected current value of the switching device 101 to a voltage signal to output the voltage signal to a comparator 108 according to the current value of the switching device 101.
Reference numeral 111 denotes a feedback signal control circuit which converts a current signal inputted to the FB terminal to a voltage signal to output the signal to the comparator 108. The comparator 108 outputs a signal to the reset terminal of a RS flip-flop circuit 110 when the output signal from the feedback signal control circuit 111 becomes equal to the output signal from the drain current detection circuit 106.
Reference numeral 112 denotes a clamp circuit which determines the maximum value of the output signal of the feedback signal control circuit 111. Since the clamp circuit 112 determines the maximum value of the current flowing into the switching device 101, the clamp circuit 112 has the function of protecting the switching device 101 from overcurrent and limits the maximum value of the primary drain current, which limits a maximum power to be supplied to a secondary load and determines an overload protection level.
Reference numeral 109 denotes an oscillation circuit, which outputs a maximum duty-cycle signal 109A, which determines the maximum duty cycle of the switching device, 101, and a clock signal 109B which determines the oscillation frequency of the switching device 101. The maximum duty-cycle signal 109A is input to the NAND circuit 105, and the clock signal 109B is input to the set terminal of the RS flip-flop circuit 110.
The output signal of the start-up and shut-down circuit 107, the maximum duty-cycle signal 109A, and an output signal of the RS flip-flop circuit 110 are input to the NAND circuit 105. The output signal of the NAND circuit 105 is input to a gate drive circuit 104 to control the switching operation of the switching device 101.
Reference numeral 140 denotes a transformer, which has a primary winding 140A, a secondary winding 140B, and a primary auxiliary winding 140C.
A rectifying-smoothing circuit composed of a diode 131 and a capacitor 132 is connected to the primary auxiliary winding 140C and is used as the auxiliary power supply section of the semiconductor device 130 used for input to VCC.
Reference numeral 133 denotes a capacitor for stabilizing VDD.
Reference numeral 135 denotes a control signal transmission circuit which transmits a control signal from the secondary side to the primary side and which is composed of a phototransistor 135A and a photodiode 135B. The collector of the phototransistor 135A is connected to FB, and the emitter of the phototransistor 135A is connected to GND.
A rectifying-smoothing circuit composed of a diode 150 and a capacitor 151 is connected to the secondary winding 140B and is further connected to the photodiode 135B, a secondary control circuit 158, and a load 157.
The secondary control circuit 158 includes a shunt regulator 152, resistors 154, 155, and 156, and a capacitor 153, supplies a voltage divided by the resistors 154 and 155 for detecting secondary output voltage VO to the reference terminal of the shunt regulator 152, and controls a current which flows into the photodiode 135B connected to the cathode of the shunt regulator so as to keep the secondary output voltage VO constant.
The operation of the switching power supply unit having such a configuration will be described with reference to FIGS. 8 and 9. FIG. 9 is a time chart of the operation waveform of each section shown in FIG. 8.
In FIG. 8, a direct-current voltage VIN formed by rectifying and smoothing commercial alternating-current power supply voltage, for instance, is supplied to the input terminal. VIN is applied to the DRAIN terminal of the semiconductor device 130 via the primary winding 140A of the transformer 140. Then, a starting current formed at the start-up constant current source 103 flows to charge the capacitor 132 connected to VCC via the switch 102A of the regulator 102, thereby the voltage at VCC is raised. Besides, since the switch 102B of the regulator 102 operates so as to make the voltage at VDD constant, the capacitor 133 connected to VDD via the switch 102B is charged by part of the starting current, thereby the voltage at VDD is also raised.
When the voltage at VCC rises to reach a starting voltage set at the start-up and shut-down circuit 107, the switching operation of the switching device 101 is started, following which energy is supplied to each winding of the transformer 140, so that current flows into the secondary winding 140B and the primary auxiliary winding 140C.
The current flowing through the secondary winding 140B is rectified by the diode 150 and smoothed by the capacitor 151 into a direct-current power, so that the power is supplied to the load 157. The output voltage VO rises gradually through the switching operation repeated. When the output voltage VO reaches a voltage set by the output voltage detecting resistors 154 and 155, the current which flows into the photodiode 135B is increased by a signal from the secondary control circuit 158.
Then, a current flowing into the phototransistor 135A increases, and a current flowing out of the terminal FB also increases.
When the FB terminal current IFB increases, a voltage VFBO inputted to the comparator 108 decreases, so that the drain current IDS flowing into the switching device 101 becomes small. As a result, the output voltage VO is stabilized by such an application of negative feedback.
The current flowing into the primary auxiliary winding 140 is rectified by the diode 131 and smoothed by the capacitor 132. Moreover, the current is utilized as an auxiliary power for the semiconductor device 130 to be supplied to the VCC terminal. When once the voltage at VCC reaches the starting voltage, the switch 102A of the regulator 102 is turned OFF, so that the current of the semiconductor device after the starting is supplied from the primary auxiliary winding 140C. Since the polarity of the primary auxiliary winding 140C is the same as that of the secondary winding 140B, the voltage at VCC is proportional to the output voltage VO.
When the output current IO flowing into the load 157 decreases after the stabilization of the output voltage VO, the feedback current IFB increases, the voltage VFBO inputted to the comparator 108 decreases, and the drain current flowing into the switching device 101 becomes small.
Also, when the output current IO flowing into the load 157 increases, the feedback current IFB decreases, the voltage VFBO inputted to the comparator 108 rises, and the drain current flowing into the switching device 101 becomes large as IO increases. When VFBO increases and then reaches a voltage defined by the clamp circuit 112, the overcurrent protecting function is performed, and the drain current is clamped by a constant current ILIMIT.
As described above, the maximum value of the primary drain current is fixed, so that a maximum power suppliable to the secondary load is limited. However, there come up problems that when the maximum power limited under peak load is applied using ILIMIT, the power is too high as the overload protection level under normal load, and when ILIMIT is applied as the overload protection level under normal load, power cannot be sufficiently supplied to the load under peak load.
FIG. 10 is a circuit diagram of another conventional switching power supply unit having an overload protecting function as a power supply. FIG. 10 is different from FIG. 8 in that an output current detection resistor 159, an overcurrent detection circuit 160, an overcurrent signal transmission circuit 136 are provided. In FIG. 10, when an output current IO exceeds a constant value, the current flowing into a photodiode 136B increases, and then the current flows from a power supply voltage terminal VDD into GND via a phototransistor 136A. Thereafter, the voltage at the VDD terminal decreases, a stop signal is output from a start-up and shut-down circuit 107, the switching operation of a switching device 101 stops, and the overload protecting function operates as the power supply, so that it becomes possible to realize the overload protecting function as the power supply when its drain current is smaller than ILIMIT. That is, when a time delay is provided for the detection at the overcurrent detection circuit 160, the protection against the peak load can be gained by ILIMIT, and the overload protection during normal operation can be gained by the drain current smaller than ILIMIT. In the configuration shown in FIG. 10, however, the element count increases inevitably.
As conventional techniques of varying an overcurrent detection level according to load variations, there have been methods for performing overcurrent protecting operation through the provisions of an overcurrent detecting system commensurate-with maximum rated loads and of another overcurrent detecting system commensurate with loads lower than the maximum rated loads. For instance, the configurations are described in JP-A No. 6-38518.
Generally, it is necessary for switching power supply units to have a protective function under overload. When a peak load condition is imposed as a load condition, it is desired that the overload protection level under normal load be set in a manner that the protective function is activated under overload but not activated under peak load. Because of this, protective functions commensurate with loads are generally provided.
Furthermore, to cope with such a provision, it becomes necessary to take additional measures such as the stop of the primary switching operation through the detection of the secondary output current, so that there also come up problems of increases in production cost and element count and of complex power supply circuit configuration.